As a professional sleep consultant, I hear the term “regression” used in regards to just about every imaginable circumstance.
https://www.babywinkz.com/the-four-month-sleep-regression/
Essentially, if your baby doesn’t sleep well for a couple of nights, parents start dropping the ‘R’ word.
Some people subscribe to the idea that there’s an eight month regression, a 9 month regression, a 1 year regression, as well as teething regressions, growth spurt regressions, and so on.
Others see these as simple hiccups caused by extenuating circumstances.